Conventionally, ocular optical systems for cameras which include two lens groups containing a negative lens and a positive lens arranged in order from the focusing glass side with one of the lenses moved along the optical axis for focusing are known as ocular optical systems for viewfinders used in single-lens reflex cameras.
In a conventional ocular optical system for a viewfinder which is formed of two groups containing two lenses including a negative lens and a positive lens, optical materials with relatively low refractive indexes are used; however, magnification and eye relief (hereafter defined as a distance on the optical axis between the surface closest to the eye point and the eye point) are the main focus when one considers that the refractive power of each lens tends to be increased. As a result, such an optical system is required to have a structure such that the distance between the focusing glass and the ocular optical system along the optical axis, that is the optical distance occupied by an erecting optical system formed of a pentaprism or pentamirror, is shortened.
Shortening of the optical distance of the erecting optical system is effective for minimizing the size of a camera. However, when the optical distance of the erecting optical system is set extremely short, a sufficient field of view is not obtained. Additionally, if the optical distance is set too short it tends to result in an unexpected crossing between optical members for display within the field of view of the viewfinder and the observation light beam. On the other hand, when the optical distance of the erecting optical system is set too long with the objective of obtaining a sufficient field of view and to avoid the unexpected crossing between the optical members for display and the observation light beam, the range of visibility compensation (eyesight correction factor, in diopters) of the ocular optical system for a viewfinder is extremely deviated to the positive side such that it is difficult to maintain sufficient range of visibility including negative visibility on the near vision side.
In a conventional ocular optical system, large coma aberrations are caused in the vicinity of an image plane due to the strong refractive power of each lens. As a result, an objective lens makes it difficult to observe the periphery of an object image formed on the focusing glass or its vicinity.